wafer alignment system

ABSTRACT

A system is provided for producing an integrated circuit using a stepper and a scanner in successive stages. Calibration data developed for the transfer of a wafer from the stepper to the scanner while maintaining the same orientation is transformed, and the transformed data is used to align a rotated wafer on the scanner.

FIELD OF THE INVENTION

[0001] The present invention relates to photolithography processes formanufacturing integrated circuits. More particularly, the inventionrelates to a method of aligning wafers for successive stepping andscanning stages of a photolithographic process.

BACKGROUND OF THE INVENTION

[0002] Photolithography is used to manufacture integrated circuits byexposing a suitably prepared wafer to light passing through a mask. Theentire wafer can be exposed at once. Often, however, separate sub-areasof a wafer are successively exposed in a stepping process, or a band oflight is directed synchronously across a mask and a region of a wafer ina scanning process. Alignment is critically important when multiplephotolithographic processes are used to manufacture an integratedcircuit.

[0003] Alignment refers to, among other things, the process ofregistering a mask to a wafer. Many methods of alignment are known. Inone method, a wafer is carried on a fixture called a wafer stage. Thewafer is indexed to the wafer stage by a notch in its periphery and thewafer stage is supported by a movable carriage. The carriage positionsthe wafer stage as part of stepping and/or scanning processes.

[0004] Mirrors are typically affixed to the wafer stage and as the waferstage is moved interferometers focused on the mirrors precisely locatethe wafer stage to align the wafer stage with the appropriate mask andlight source. Typically the wafer stage is rectilinear. Therefore, onlytwo sets of two mirrors, one set parallel to the x-axis and one setparallel to the y-axis, are required to appropriately locate the waferstage in the x-y plane.

[0005] An example of a photolithographic process including stepping andscanning steps is illustrated in FIGS. 1-3. In FIG. 1, a light sourceand mask are aligned to expose a first region 1 of a wafer A. In FIG. 2,the light source and mask are aligned to expose a second region 2 ofwafer A. This constitutes a two-step stepping process. A scanningprocess then commences. In the first step of the scanning process, amask is aligned with a third region 3 of wafer A, and a light sourcetraverses the mask exposing region 3 in FIG. 3.

[0006] Alignment of the masks used in the scanning process with theexisting stepped regions is critical. This alignment becomes moredifficult when the scanning process is completed on a different machinefrom the stepping process. Moreover, the surfaces of the mirrors used toalign the wafer stage are not completely flat, and mirror imperfectionswill affect alignment when critical dimensions are small. The mirrors,therefore, must be calibrated.

[0007] One method to accomplish this inter-machine alignment uses acalibration wafer. According to this method, a calibration wafer isplaced in a first machine, and a calibration pattern is printed by thefirst machine on the calibration wafer. The actual position of thepoints of the calibration pattern are carefully measured. Thecalibration pattern measurement data, along with the position of thecalibration wafer according to the alignment mirrors of the firstmachine, is stored in a memory.

[0008] The calibration wafer is placed in the second machine in the sameorientation as the first machine. A nominally identical calibrationpattern is printed by the second machine on the calibration wafer. Theactual position of the points of the second calibration pattern arecarefully measured. The second calibration pattern measurement data,along with the position of the calibration wafer according to thealignment mirrors of the second machine, is stored in a memory.

[0009] The first calibration pattern measurement data, first alignmentmirror position, second calibration pattern measurement data and secondalignment mirror position are processed to account for, among otherthings, the disparities of the alignment mirrors. When a productionwafer is processed in a first machine, then transferred to a secondmachine in the same orientation, the processed data from the calibrationprocess is used to adjust the position of the production wafer in thesecond machine to bring it into true alignment with the regions exposedon the production wafer by the first machine.

[0010] When scanning is done in the same linear direction as stepping,once the wafer is placed in the apparatus, its only movement will bealong the x and y axes and no rotation to change wafer orientation isnecessary. For instance, in FIG. 10, a shallow, rectangular first region1 a is exposed on a wafer C, followed by a similar second region 2 a asshown in FIG. 11. These stepping processes could be followed by onescanning process similar to those shown in FIG. 3. Sometimes, however,it is advantageous to carry out stepping and scanning processes indifferent directions with respect to a wafer. For example, as shown inFIG. 12, under certain geometries a single pass of the scanner 200 in adirection 90° to the path of the stepper 100 can expose a single region3 a covering both regions 1 a and 2 a.

[0011] Many integrated circuit manufacturing centers are not equipped toexecute stepping and scanning in different directions. In thesemanufacturing centers, the wafer must be rotated 90° to accommodatestepping passes orthogonal to scanning passes. This is illustrated inFIG. 13, where the wafer C has been rotated 90° to accommodate a region4 a scanned in the same linear direction as the stepping processes. Whenmulti-directional stepping and scanning requires a rotation of a wafer,the alignment process described above cannot be used. What is requiredthen, is a method of aligning and manufacturing a rotated productionwafer.

SUMMARY OF THE INVENTION

[0012] The invention concerns a method for aligning wafers in machinesused to manufacture integrated circuits.

[0013] In the invention, a first pattern is formed in a calibrationwafer in a first orientation in a first machine and a second pattern isformed in the calibration wafer in said first orientation in a secondmachine. Next, the difference between the first pattern and the secondpattern is measured and stored in a memory. The difference istransformed to account for a change in orientation, typically a 90°rotation.

[0014] Next, regions in a production wafer in the first orientation areprocessed in the first machine and the location of the production waferin the first machine is determined.

[0015] The production wafer is then transferred to the second machine ina second orientation, typically at a 90° rotation.

[0016] The location of the production wafer in the second machine isdetermined next, and then adjusted using the transformed difference.Finally, the production wafer is aligned in the second machine using theadjusted location data; and the regions in the production wafer areprocessed in the second machine.

[0017] In one example of the invention, the first machine is a stepperand the second machine is a scanner, each with their own processor andmemory. The scanner processor retrieves the coordinates of the cruciformpatterns, transforms them, and adjusts the alignment of the productionwafer in the scanner using the transformed coordinates.

[0018] A 90° change in the orientation of the production wafer is usefulwhen two successive regions of the production wafer are exposed in thestepper in a first direction, the scanning breadth of the scannerexceeds the length of the two successive stepped regions in the firstdirection, and a single scanning pass in a second direction exposes bothsuccessive stepped regions in the production wafer in a single scanningpass.

[0019] According to one aspect of the invention, positional differencesmay be transformed by switching the x-coordinates of the cruciformpattern in the scanner with the y-coordinates of the cruciform patternin the scanner.

[0020] The above and other advantages and features of the invention willbe more readily understood from the following detailed description ofthe invention which is provided in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a plan view of a wafer with a first stepped areaexposed.

[0022]FIG. 2 is a plan view of the wafer of FIG. 1 with a second steppedarea exposed.

[0023]FIG. 3 is a plan view of the wafer of FIGS. 1 and 2 with a firstscanned area exposed.

[0024]FIG. 4 is a partial schematic drawing of a stepper and a scanner.

[0025]FIG. 5 is a plan view of a calibration wafer with a nominalcruciform pattern.

[0026]FIG. 6 is a plan view of a calibration wafer with an actualcruciform pattern formed in a stepper.

[0027]FIG. 7 is a plan view of a calibration wafer with a second actualcruciform pattern formed in a scanner.

[0028]FIG. 8 is a plan view of a portion of the calibration wafer ofFIG. 6 and FIG. 7.

[0029]FIG. 9 is a flow chart for an integrated circuit manufacturingprocess including stepping and scanning.

[0030]FIG. 10 is a plan view of a wafer with a first stepped areaexposed.

[0031]FIG. 11 is a plan view of a wafer with a second stepped areaexposed.

[0032]FIG. 12 is a plan view of a wafer with a single scanned areaexposed.

[0033]FIG. 13 is a plan view of a wafer rotated to accommodate a singlescanning process.

[0034]FIG. 14 is a partial schematic drawing of a stepper and scannerwherein a production wafer is rotated when transferred from the stepperto the scanner.

[0035]FIG. 15 is a flow chart for an integrated circuit manufacturingprocess wherein a wafer is rotated when transferred from a stepper to ascanner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0036] As shown in FIG. 4, a calibration wafer 10 is placed in a waferstage 112 of a first, or reference machine, in this case, the stepper100. The wafer stage 112 of the stepper 100 is supported by a carriage(not shown). The carriage is capable of moving the wafer stage in the xand y directions, as indicated by the arrows 12. The wafer stage 112 hasan x-location mirror 114 attached to a side 115 parallel to the y-axisand a y-location mirror 116 attached to a side 117 parallel to thex-axis. An x-location interferometer 118 focused on the x-locationmirror 114 is attached to the frame (not shown) of the stepper 100, anda y-location interferometer 120 also attached to the frame, is focusedon the y-location mirror 116. The movement and location of the waferstage is very precisely controlled.

[0037] As shown in FIG. 5, a cruciform pattern 14 is printed on thecalibration wafer 10 consisting of points arranged in a vertical bar 16,a nominally straight line parallel to the y-axis, and a horizontal bar18, a nominally straight line parallel to the x-axis. This pattern isproduced by moving the wafer stage 112, under a light source (not shown)focused at a non-moving point on the surface of the calibration wafer10. The wafer stage 112 is then incrementally traversed through therange of the carriage in the y-direction while holding a single positionof the carriage in the x-direction as indicated by the x-locationinterferometer 118 reading of the x-location mirror 114. Next, the waferstage 112 is incrementally traversed throughout the range of thecarriage in the x-direction while holding a single position of thecarriage in the y-direction as indicated by the y-locationinterferometer 120 reading of the y-location mirror 116. Because themirrors are not perfectly flat the actual cruciform pattern 14 aproduced will be slightly curved as shown in FIG. 6.

[0038] The actual positions of the points along the nominally cruciformpattern 14 a formed on the calibration wafer 10 are precisely determinedusing the stepper metrology. The x and y coordinates of these pointsconstitute an array, x_(A), y_(A)={x_(A1), y_(A1), x_(A2), y_(A2),x_(A3), y_(A3) . . . x_(An), y_(An)}. Returning to FIG. 4, this array ofpoints is transmitted by the processor 122 of the stepper 100 to itsmemory 123.

[0039] The calibration wafer 10 is removed from the wafer stage 112 ofthe stepper 100 and is placed in the wafer stage 212 of the scanner 200.During this transfer step, the calibration wafer 10 is maintained in thesame orientation in the x-y plane with the notch 11 of the wafer 10facing right. A second nominally cruciform pattern 14 b is printed onthe calibration wafer 10 in the same manner as the pattern 14 a wasformed on the stepper 100. The second actual cruciform pattern 14 b isalso curved and is shown in FIG. 7. The first actual cruciform pattern14 a is omitted from FIG. 7 for clarity.

[0040] The actual positions of the points along the second nominallycruciform pattern 14 b formed on the calibration wafer 10 are thenprecisely determined using the scanner metrology. The x and ycoordinates of these points constitute an array, x_(B), y_(B)={x_(B1),y_(B1), x_(B2), y_(B2), x_(B3), y_(B3) . . . x_(Bn), y_(Bn)}. This arrayis transmitted by the processor 222 of the scanner 200 to its memory223.

[0041] The coordinates of the array x_(A), y_(A) stored in the steppermemory 123 are transmitted to the scanner memory 223 by any of a numberof means known in the art. A calibration array is then calculated by thescanner processor 222 using the difference between the actual cruciformpattern 14 a produced by the stepper 100 and the actual cruciform 14 bpattern produced by the scanner 200. This difference is the aggregatedifferences in the actual positions of corresponding locations on thex-axis for each point on the vertical bar 16 of the cruciform pattern 14and the actual positions of corresponding locations on the y-axis foreach point on the horizontal bar 18 of the cruciform pattern 14.

[0042] To illustrate this calculation, FIG. 8 is a top view of anenlarged part of the calibration wafer showing the part of the verticalbars 16 a, 16 b of the superimposed actual cruciform pattern 14 a of thestepper 100 and the actual cruciform pattern 14 b of the scanner 200.For each incremental position on the vertical bar 16, the horizontaldistance between the corresponding points on the cruciform pattern iscalculated. For example, for position y₁, shown in FIG. 8, thehorizontal distance x_(A1)-x_(B1) between corresponding points of theactual cruciform pattern 14 a of the stepper 100 and the actualcruciform pattern 14 b of the scanner 200 is calculated. Thiscalculation is repeated for each incremental position on the verticalbar 16, and is assembled into the vertical component of the calibrationarray (x_(A)-x_(B)), y=(x_(A1)-x_(B2)), y₁, (x_(A2)-x_(B2)), y₂, . . .(x_(An)-x_(Bn)), y_(n). This vertical calibration array accounts for thedifference in profile between the x-location mirror 114 of the stepper100 and the x-location mirror 214 of the scanner 200.

[0043] Similarly, for each incremental position on the horizontal bar,the vertical distances between the corresponding points in the cruciformpatterns are calculated, and are assembled into the horizontal componentof the calibration array x, (y_(A)-y_(B))={x₁, (y_(A1)-y_(B1)), x₂(y_(A2)-y_(B2)), . . . x_(n) (y_(AN)-y_(BA))}. The horizontalcalibration array component accounts for the difference in profilebetween the y-location mirror 116 of the stepper 100 and the y-locationmirror 216 of the scanner 200. The complete calibration array(x_(A)-x_(B)), y, x (y_(A)-y_(B)) includes both the vertical andhorizontal components.

[0044] During the manufacture of an integrated circuit according to thestepping and scanning pattern of FIGS. 1-3, the calibration array isused to determine and control the position of a production wafer 22 inthe integrated circuit manufacturing center of FIG. 4, as follows: Theproduction wafer 22 is placed in the wafer stage 112 of the stepper 100.The wafer stage 112, light source, lens, and mask are aligned to produceregion 1 and region 1 is exposed. During alignment, location data forthe wafer stage 112 is obtained using the alignment mirrors 114, 116 andinterferometers 118, 120 of the stepper. The wafer stage location isprocessed by the processor 122 of the stepper 100, and is stored inmemory 123. After exposure of region 1, the wafer stage 112 moves toregion 2 and the mask is aligned and region 2 is exposed.

[0045] For the next layer, the production wafer 22 is removed from thestepper 100 and placed in the wafer stage 212 of the scanner 220. Duringthis transfer step, the production wafer 22 is maintained in the sameorientation in the x-y plane. The wafer stage 212, light source, lensand mask of the scanner are aligned in order to commence scanning ofregion 3 of the production wafer. In order for scanned sub-area 3 toalign with sub-areas 1 and 2 previously produced, the scanner processor222 transforms the location data obtained from the stepper 100 using thecalibration array according to mathematical models known in the art, andthe scanner 200 locates the wafer stage 212 according to the transformedlocation data using the alignment mirrors 214, 216 and interferometers218, 220. The transformed location data used to align wafer stage 212accommodates the imperfections of the location mirrors of the waferstages of the stepper 100 and scanner 200. By using the transformedlocation data the wafer stage 212 can be correctly positioned so thatthe scanning process aligns with the previously exposed regions from thestepping process.

[0046] This manufacturing process may be illustrated using the flowchart set forth in FIG. 9. A production wafer 22 is placed in the waferstage 112 of stepper 100 at step 400. Next, the location of the waferstage is determined using the interferometers 118, 120 and mirrors 114,116 of the stepper 100 at step 402. This location data of the waferstage 112 of the stepper constitutes stepper array x_(PA), y_(PA). Thestepper location array x_(PA), y_(PA) is processed by the processor 122at step 404 and stored at the stepper memory 123 at step 406. At step408, the photolithographic manufacturing process of the stepper 100 iscompleted. At step 410, the production wafer 22 is transferred from thewafer stage 112 of the stepper 100 and placed in the wafer stage 212 ofthe scanner 200. During this transfer step, the production wafer 22maintains the same orientation in the x-y plane. In FIG. 4 thisorientation is with the notch 23 facing up.

[0047] In order to align the wafer stage 212 in the scanner 200, thestepper location array x_(PA), y_(PA) of the wafer stage 112 of thestepper 100 is transformed by the calibration array. Specifically, thescanner processor 222 retrieves stepper location array x_(PA), y_(PA)from the memory 223 at step 412 and retrieves the calibration array fromthe memory 223 at step 414. At step 416, the calibration array(x_(A)-x_(B)), y, x, (y_(A)-y_(B)) is used to transform the stepperlocation array x_(PA), y_(PA) to produce a scanner location array x_(PB)y_(PB). The scanner location array x_(PB), y_(PB) is used to align thewafer stage 212 of the scanner 200 in step 420. The scanner 200completes its photolithographic manufacturing process at step 422.

[0048] Under the improved alignment method for accommodating rotatedwafers during the manufacture of an integrated circuit, an existingcalibration array obtained using a calibration wafer 10 that is notrotated, is modified and used to determine and control the position of aproduction wafer 23 that is rotated when transferred from a stepper to ascanner. As shown in FIG. 14, the production wafer 23 is placed in thewafer stage 512 of the stepper 500 with its notch 24 facing in a firstdirection (x). The wafer stage 512, light source, lens, and mask (notshown) are aligned to produce sub-area 1 and sub area 1 is exposed asshown in FIG. 10. During alignment, location data x_(NA), y_(NA) for thewafer stage 512 is obtained using the alignment mirrors 514, 516 andinterferometers 518, 520 of the stepper 500. This location is processedby the processor 522 of the stepper 500 at step 504 and transmitted tothe stepper memory 523. After exposure of sub-area 1, the wafer stage512 moves to sub-area 2 and the mask is aligned and sub-area 2, FIG. 11,is exposed.

[0049] For the next layer, after other processes, the production wafer23 is placed in the wafer stage 612 of the scanner 620. During thisstep, the production wafer 23 is rotated 90° in the x-y plane, so thatits notch 24 faces in a second direction (y), to accommodate a singlescanning pass. In the illustrated embodiment, the second direction (y)is orthogonal to the first direction (x). The present invention shouldnot be limited, however, to the preferred embodiments shown anddescribed in detail herein. Because of the rotation of the productionwafer 23, the calibration array obtained with a calibration wafer thatwas not rotated is modified by switching the sub-array x_(B) for thesub-array y_(B). Substituting y_(B) for x_(B) in the vertical componentof the calibration array, (x_(A)-y_(B)), y, accounts for the differencein profile between the x-location mirror 514 of the stepper 500 and they-location mirror 616 of the scanner 600. Similarly, substituting x_(B)for y_(B) in the horizontal component of the calibration array, x,(y_(A)-x_(B)), accounts for the difference in profile between they-location mirror 516 of the stepper 500 with the x-location mirror 614of the scanner 600. These modifications effect a switch of the verticalbar 16 a with the horizontal bar 18 b of the actual cruciform patternproduced in the calibration wafer 10 by the scanner 600. The complete,modified calibration array is represented by (x_(A)-y_(B)), y, x,(y_(A)-x_(B)).

[0050] The wafer stage 612, light source, lens and mask of the scanner600 are aligned to commence scanning of sub-area 3 of the productionwafer. In order for scanned sub-area 3 to align with sub-areas 1 and 2previously produced, the scanner processor 622 transforms the locationdata x_(NA), y_(NA) obtained from the stepper 500 using the modifiedcalibration array and mathematical models known in the art. Then thetransformed location x_(NB), y_(NB) data is used by the scanner 600 tolocate the wafer stage 612 according to the transformed location datax_(NB), y_(NB) using the alignment mirrors 614, 616 and interferometers618, 620. The transformed location data x_(NB), y_(NB) correctly locatesthe wafer stage 612 so that the scanning step aligns with the previouslyexposed areas from the stepping process.

[0051] Referring now to FIG. 15, a production wafer 23 is placed in thewafer stage 512 of stepper 500 at step 800. Next, the location of thewafer stage is determined using the interferometers 518, 520 and mirrors514, 516 of the stepper 500 at step 802. This location data of the waferstage 512 of the stepper 500 constitutes an array x_(NA), y_(NA). Thestepper location array data x_(NA), y_(NA) is processed by the processor522 of the stepper 500 at step 804 and is stored in memory 523 at step806. At step 808, the photolithographic manufacturing process of thestepper 500 is completed. At step 810 a, the production wafer 23 isremoved from the wafer stage 512 of the stepper 500, rotated 90° at step810 b and placed in the wafer stage 612 of the scanner 600 at step 810c. In FIG. 14, the orientation of the production wafer 23 changes fromthe notch facing in the first direction (x) in the stepper 500 to facingin the second direction (y) in the scanner 600.

[0052] To align the wafer stage 612 in the scanner 600, the locationdata x_(NA), y_(NA) of the wafer stage 512 of the stepper 500 istransformed by the modified calibration array. Specifically, the scannerprocessor 622 retrieves the stepper location array data x_(NA), y_(NA)from the memory 623 at step 812 and retrieves the modified calibrationarray from the memory 623 at step 814. At step 816, the modifiedcalibration array (x_(A)-y_(B)), y, x, (y_(A)-x_(B)) is used totransform the stepper location array x_(NA), y_(NA) to produce a scannerlocation sub-array x_(NB), y_(NB). The scanner location array datax_(NB), y_(NB) is used to align the wafer stage 612 of the scanner 600in step 820. The scanner 600 completes its photolithographicmanufacturing process at step 822.

[0053] The invention provides a method of transforming calibration datato accommodate the rotation of production wafers in successive steppingand scanning stages in the manufacture of integrated circuits.Variations of the disclosed embodiment will be readily apparent to thoseskilled in the art. For instance, different stepping and scanningprocesses could be used to practice the invention and differentmathematical nomenclature could be used. In addition the variousprocessors and memory devices could be distributed differently than thecomponents of the manufacturing center described. Accordingly, it is tobe understood that although the present invention has been describedwith reference to exemplary embodiments, various modifications may bemade without departing from the spirit or scope of the invention whichis defined solely by the claims appended hereto.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of transforming calibration data in awafer production apparatus, said method comprising the steps of:acquiring calibration data representative of the alignment of a secondmachine with respect to a first machine; and exchanging firstcoordinates of said calibration data with second coordinates of saidcalibration data.
 2. The method of claim 1, further comprising rotatinga production wafer with respect to a calibration wafer.
 3. The method ofclaim 2, wherein said rotation of said production wafer is 90°.
 4. Themethod of claim 1, wherein said first coordinates are x-coordinates andsaid second coordinates are y-coordinates.
 5. A method of aligning aproduction wafer comprising the steps of: retrieving calibration datafor the alignment of a second machine with respect to a first machine;retrieving alignment data for the production wafer in the first machine;and transforming said calibration data by switching coordinates.
 6. Themethod of claim 5, wherein said production wafer is rotated 90° withrespect to the position of a calibration wafer in said second machine.7. The method of claim 5, wherein said coordinates are x-coordinates andy-coordinates.
 8. The method of claim 5, wherein said first machine is astepper, and said second machine is a scanner.
 9. The method of claim 8,further comprising the step of storing said transformed calibration datain said first machine.
 10. The method of claim 9, wherein said firstmachine uses said transformed calibration data to adjust the alignmentof the production wafer.
 11. The method of claim 10, wherein twosuccessive areas of the production wafer are exposed in the stepper. 12.A method of aligning a production wafer comprising the steps of:calibrating the wafer stage of the second machine to the wafer stage ofthe first machine; transforming the data from said calibration;measuring the location of a production wafer in the first machine;transferring the production wafer to the second machine; adjusting thelocation of the production wafer in the second machine using saidtransformed data.
 13. The method of claim 12, wherein said alignment ofthe production wafer in the second machine is rotated 90° with respectto said calibration.
 14. The method of claim 12, wherein said firstmachine is a stepper, and said second machine is a scanner.
 15. Themethod of claim 12, further comprising the step of storing saidtransformed calibration data in said first machine.
 16. The method ofclaim 15, wherein said first machine adjusts the location of theproduction wafer using the transformed data from the calibration.
 17. Amethod of aligning wafers in machines used to manufacture an integratedcircuit, comprising the steps of: measuring the difference in locationfrom a location in a first machine to a nominally identical location ina second machine using a first wafer maintained in the same orientation;transforming said difference in said locations to account for a changein wafer orientation; measuring the location of a second wafer in thefirst machine; transferring the second wafer to the second machine in adifferent orientation; adjusting the location of the second wafer in thesecond machine using said transformed differences.
 18. The method ofclaim 17, wherein said first wafer is a calibration wafer.
 19. Themethod of claim 17, wherein said second wafer is a production wafer. 20.The method of claim 18, wherein said difference is measured by comparingpatterns formed in the calibration wafer by the first machine and thesecond machine.
 21. The method of claim 20, wherein the pattern iscruciform.
 22. The method of claim 18, wherein said calibration wafer ismounted in a wafer stage and said location of the calibration wafer isdetermined by measuring the location of the wafer stage.
 23. The methodof claim 22, wherein the wafer stage has mirrors, and the location ofthe wafer stage is measured using interferometers mounted in said firstand second machines.
 24. The method of claim 17, wherein said firstmachine is a stepper.
 25. The method of claim 17, wherein said secondmachine is a scanner.
 26. The method of claim 25, wherein two successiveareas of the production wafer are exposed in the stepper.
 27. The methodof claim 25, further comprising the step of storing said data in saidstepper.
 28. The method of claim 27, wherein said stepper uses saidstored data to adjust the location of the second wafer.
 29. A method ofmanufacturing an integrated circuit, comprising the steps of: forming afirst cruciform pattern in a calibration wafer in a first orientation ina first machine; forming a second cruciform pattern in said calibrationwafer in said first orientation in a second machine; measuring thedifference between said first cruciform pattern and said secondcruciform pattern; storing said difference in a memory; transformingsaid difference to account for a change in orientation; processingsub-areas in a production wafer in said first orientation in said firstmachine; determining the location of said production wafer in said firstmachine; transferring said production wafer to said second machine in asecond orientation; adjusting said location using said difference;aligning said production wafer in said second machine using saidadjusted location data; and processing sub-areas in said productionwafer in said second machine.
 30. The method of claim 29, wherein saidfirst machine is a stepper and said second machine is a scanner.
 31. Themethod of claim 29, wherein said second orientation is rotated 90° fromsaid first orientation.
 32. The method of claim 31, wherein twosuccessive areas of the production wafer are exposed in the stepper. 33.The method of claim 29, wherein said difference is transformed byswitching coordinates of the cruciform pattern.
 34. The method of claim30, further comprising storing said data in said stepper.
 35. The methodof claim 34, wherein said stepper uses said stored data to adjust thealignment of the production wafer using the transformed coordinates. 36.The method of claim 33, wherein said difference is the array(x_(A)-x_(B)), y, x, (y_(A)-y_(B)), and said transformed difference isrepresented by the array (x_(A)-y_(B)), y, x, (y_(A)-x_(B)).
 37. Asystem for transforming calibration data in a wafer productionapparatus, said system comprising: a device for acquiring calibrationdata representative of the alignment of a second machine with respect toa first machine; and a device for exchanging first coordinates of saidcalibration data with second coordinates of said calibration data.